[PATCH v24 12/34] ASoC: doc: Add documentation for SOC USB

@@ -18,3 +18,4 @@ The documentation is spilt into the following sections:-
    jack
    dpcm
    codec-to-codec
+   usb

@@ -0,0 +1,435 @@
+================
+ASoC USB support
+================
+
+Overview
+========
+In order to leverage the existing USB sound device support in ALSA, the
+ASoC USB APIs are introduced to allow for the entities to communicate
+with one another.
+
+One potential use case would be to support USB audio offloading, which is
+an implementation that allows for an external DSP on the SoC to handle the
+transfer of audio data over the USB bus.  This would let the main
+processor to stay in lower power modes for longer durations.  The following
+is an example design of how the ASoC and ALSA pieces can be connected
+together to achieve this:
+
+::
+
+               USB                   |            ASoC
+                                     |  _________________________
+                                     | |   ASoC Platform card    |
+                                     | |_________________________|
+                                     |         |           |
+                                     |      ___V____   ____V____
+                                     |     |ASoC BE | |ASoC FE  |
+                                     |     |DAI LNK | |DAI LNK  |
+                                     |     |________| |_________|
+                                     |         ^  ^        ^
+                                     |         |  |________|
+                                     |      ___V____    |
+                                     |     |SOC-USB |   |
+     ________       ________               |        |   |
+    |USB SND |<--->|USBSND  |<------------>|________|   |
+    |(card.c)|     |offld   |<----------                |
+    |________|     |________|___     | |                |
+        ^               ^       |    | |    ____________V_________
+        |               |       |    | |   |IPC                   |
+     __ V_______________V_____  |    | |   |______________________|
+    |USB SND (endpoint.c)     | |    | |              ^
+    |_________________________| |    | |              |
+                ^               |    | |   ___________V___________
+                |               |    | |->|audio DSP              |
+     ___________V_____________  |    |    |_______________________|
+    |XHCI HCD                 |<-    |
+    |_________________________|      |
+
+
+SOC USB driver
+==============
+Structures
+----------
+``struct snd_soc_usb``
+
+  - ``list``: list head for SND SOC struct list
+  - ``component``: reference to ASoC component
+  - ``num_supported_streams``: number of supported concurrent sessions
+  - ``connection_status_cb``: callback to notify connection events
+  - ``get_offload_dev``: callback to fetch selected USB sound card/PCM device
+  - ``priv_data``: driver data
+
+The snd_soc_usb structure can be referenced using the ASoC platform card
+device, or a USB device (udev->dev).  This is created by the ASoC BE DAI
+link, and the USB sound entity will be able to pass information to the
+ASoC BE DAI link using this structure.
+
+``struct snd_soc_usb_device``
+
+  - ``card_idx``: sound card index associated with USB sound device
+  - ``pcm_idx``: pcm device index associated with the USB sound device
+  - ``chip_idx``: USB sound chip array index
+  - ``num_playback``: number of playback streams
+  - ``num_capture``: number of capture streams
+  - ``list``: list head for the USB sound device list
+
+The struct snd_soc_usb_device is created by the USB sound offload driver.
+This will carry basic parameters/limitations that will be used to
+determine the possible offloading paths for this USB audio device.
+
+Functions
+---------
+.. code-block:: rst
+
+	const char *snd_soc_usb_get_components_tag(bool playback);
+..
+
+  - ``playback``: direction of audio stream
+
+**snd_soc_usb_get_components_tag()** returns the tag used for describing if USB
+offloading is supported for appending to a sound card's components string.
+
+Returns a tag based on the direction of the audio stream.
+
+.. code-block:: rst
+
+	int snd_soc_usb_find_format(int card_idx, struct snd_pcm_hw_params *params,
+			int direction)
+..
+
+  - ``card_idx``: the index into the USB sound chip array.
+  - ``params``: Requested PCM parameters from the USB DPCM BE DAI link
+  - ``direction``: capture or playback
+
+**snd_soc_usb_find_format()** ensures that the requested audio profile being
+requested by the external DSP is supported by the USB device.
+
+Returns 0 on success, and -EOPNOTSUPP on failure.
+
+.. code-block:: rst
+
+	int snd_soc_usb_connect(struct device *usbdev, struct snd_soc_usb_device *sdev)
+..
+
+  - ``usbdev``: the usb device that was discovered
+  - ``sdev``: capabilities of the device
+
+**snd_soc_usb_connect()** notifies the ASoC USB DCPM BE DAI link of a USB
+audio device detection.  This can be utilized in the BE DAI
+driver to keep track of available USB audio devices.  This is intended
+to be called by the USB offload driver residing in USB SND.
+
+Returns 0 on success, negative error code on failure.
+
+.. code-block:: rst
+
+	int snd_soc_usb_disconnect(struct device *usbdev, struct snd_soc_usb_device *sdev)
+..
+
+  - ``usbdev``: the usb device that was removed
+  - ``sdev``: capabilities to free
+
+**snd_soc_usb_disconnect()** notifies the ASoC USB DCPM BE DAI link of a USB
+audio device removal.  This is intended to be called by the USB offload
+driver that resides in USB SND.
+
+.. code-block:: rst
+
+	void *snd_soc_usb_find_priv_data(struct device *usbdev)
+..
+
+  - ``usbdev``: the usb device to reference to find private data
+
+**snd_soc_usb_find_priv_data()** fetches the private data saved to the SOC USB
+device.
+
+Returns pointer to priv_data on success, NULL on failure.
+
+.. code-block:: rst
+
+	int snd_soc_usb_setup_offload_jack(struct snd_soc_component *component)
+..
+
+  - ``component``: ASoC component to add the jack
+
+**snd_soc_usb_setup_offload_jack()** is a helper to add a sound jack control to
+the platform sound card.  This will allow for consistent naming to be used on
+designs that support USB audio offloading.
+
+Returns 0 on success, negative otherwise.
+
+.. code-block:: rst
+
+	int snd_soc_usb_disable_offload_jack(struct snd_soc_component *component)
+..
+
+  - ``component``: ASoC component to disable the jack
+
+**snd_soc_usb_disable_offload_jack()** is a helper to disable a sound jack control
+on the platform sound card.
+
+Returns 0 on success, negative otherwise.
+
+.. code-block:: rst
+
+	int soc_usb_get_offload_device(struct device *dev, int card, int pcm,
+								enum snd_soc_usb_kctl route)
+..
+
+  - ``dev``: USB device to look up offload path mapping
+  - ``card``: USB sound card index
+  - ``pcm``: USB sound PCM device index
+  - ``route``: kcontrol type
+
+**soc_usb_get_offload_device()** is a registered callback to the USB BE DAI link
+to fetch the information about the mapped ASoC devices for executing USB audio
+offload for the device.
+
+Returns 0 on success, negative otherwise.
+
+.. code-block:: rst
+
+	struct snd_soc_usb *snd_soc_usb_allocate_port(struct snd_soc_component *component,
+			int num_supported_streams, void *data);
+..
+
+  - ``component``: DPCM BE DAI link component
+  - ``num_supported_streams``: number of active streams supported by external DSP
+  - ``data``: private data
+
+**snd_soc_usb_allocate_port()** allocates a SOC USB device and populates standard
+parameters that is used for further operations.
+
+Returns a pointer to struct soc_usb on success, negative on error.
+
+.. code-block:: rst
+
+	void snd_soc_usb_free_port(struct snd_soc_usb *usb);
+..
+
+  - ``usb``: SOC USB device to free
+
+**snd_soc_usb_free_port()** frees a SOC USB device.
+
+.. code-block:: rst
+
+	void snd_soc_usb_add_port(struct snd_soc_usb *usb);
+..
+
+  - ``usb``: SOC USB device to add
+
+**snd_soc_usb_add_port()** add an allocated SOC USB device to the SOC USB framework.
+Once added, this device can be referenced by further operations.
+
+.. code-block:: rst
+
+	void snd_soc_usb_remove_port(struct snd_soc_usb *usb);
+..
+
+  - ``usb``: SOC USB device to remove
+
+**snd_soc_usb_remove_port()** removes a SOC USB device from the SOC USB framework.
+After removing a device, any SOC USB operations would not be able to reference the
+device removed.
+
+How to Register to SOC USB
+--------------------------
+The ASoC DPCM USB BE DAI link is the entity responsible for allocating and
+registering the SOC USB device on the component bind.  Likewise, it will
+also be responsible for freeing the allocated resources.  An example can
+be shown below:
+
+.. code-block:: rst
+
+	static int q6usb_component_probe(struct snd_soc_component *component)
+	{
+		...
+		data->usb = snd_soc_usb_allocate_port(component, 1, &data->priv);
+		if (!data->usb)
+			return -ENOMEM;
+
+		usb->connection_status_cb = q6usb_alsa_connection_cb;
+
+		ret = snd_soc_usb_add_port(usb);
+		if (ret < 0) {
+			dev_err(component->dev, "failed to add usb port\n");
+			goto free_usb;
+		}
+		...
+	}
+
+	static void q6usb_component_remove(struct snd_soc_component *component)
+	{
+		...
+		snd_soc_usb_remove_port(data->usb);
+		snd_soc_usb_free_port(data->usb);
+	}
+
+	static const struct snd_soc_component_driver q6usb_dai_component = {
+		.probe = q6usb_component_probe,
+		.remove = q6usb_component_remove,
+		.name = "q6usb-dai-component",
+		...
+	};
+..
+
+BE DAI links can pass along vendor specific information as part of the
+call to allocate the SOC USB device.  This will allow any BE DAI link
+parameters or settings to be accessed by the USB offload driver that
+resides in USB SND.
+
+USB Audio Device Connection Flow
+--------------------------------
+USB devices can be hotplugged into the USB root hub at any point in time.
+The BE DAI link should be aware of the current state of the physical USB
+port, i.e. if there are any USB devices with audio interface(s) connected.
+connection_status_cb() can be used to notify the BE DAI link of any change.
+
+This is called whenever there is a USB SND interface bind or remove event,
+using snd_soc_usb_connect() or snd_soc_usb_disconnect():
+
+.. code-block:: rst
+
+	static void qc_usb_audio_offload_probe(struct snd_usb_audio *chip)
+	{
+		...
+		snd_soc_usb_connect(usb_get_usb_backend(udev), sdev);
+		...
+	}
+
+	static void qc_usb_audio_offload_disconnect(struct snd_usb_audio *chip)
+	{
+		...
+		snd_soc_usb_disconnect(usb_get_usb_backend(chip->dev), dev->sdev);
+		...
+	}
+..
+
+In order to account for conditions where driver or device existence is
+not guaranteed, USB SND exposes snd_usb_rediscover_devices() to resend the
+connect events for any identified USB audio interfaces.  Consider the
+the following situtation:
+
+	**usb_audio_probe()**
+	  | --> USB audio streams allocated and saved to usb_chip[]
+	  | --> Propagate connect event to USB offload driver in USB SND
+	  | --> **snd_soc_usb_connect()** exits as USB BE DAI link is not ready
+
+	BE DAI link component probe
+	  | --> DAI link is probed and SOC USB port is allocated
+	  | --> The USB audio device connect event is missed
+
+To ensure connection events are not missed, **snd_usb_rediscover_devices()**
+is executed when the SOC USB device is registered.  Now, when the BE DAI
+link component probe occurs, the following highlights the sequence:
+
+	BE DAI link component probe
+	  | --> DAI link is probed and SOC USB port is allocated
+	  | --> SOC USB device added, and **snd_usb_rediscover_devices()** runs
+
+	**snd_usb_rediscover_devices()**
+	  | --> Traverses through usb_chip[] and for non-NULL entries issue
+	  |     **connection_status_cb()**
+
+In the case where the USB offload driver is unbound, while USB SND is ready,
+the **snd_usb_rediscover_devices()** is called during module init.  This allows
+for the offloading path to also be enabled with the following flow:
+
+	**usb_audio_probe()**
+	  | --> USB audio streams allocated and saved to usb_chip[]
+	  | --> Propagate connect event to USB offload driver in USB SND
+	  | --> USB offload driver **NOT** ready!
+
+	BE DAI link component probe
+	  | --> DAI link is probed and SOC USB port is allocated
+	  | --> No USB connect event due to missing USB offload driver
+
+	USB offload driver probe
+	  | --> **qc_usb_audio_offload_init()**
+	  | --> Calls **snd_usb_rediscover_devices()** to notify of devices
+
+USB Offload Related Kcontrols
+=============================
+Details
+-------
+A set of kcontrols can be utilized by applications to help select the proper sound
+devices to enable USB audio offloading.  SOC USB exposes the get_offload_dev()
+callback that designs can use to ensure that the proper indices are returned to the
+application.
+
+Implementation
+--------------
+
+**Example:**
+
+  **Sound Cards**:
+
+	::
+
+	  0 [SM8250MTPWCD938]: sm8250 - SM8250-MTP-WCD9380-WSA8810-VA-D
+						SM8250-MTP-WCD9380-WSA8810-VA-DMIC
+	  1 [Seri           ]: USB-Audio - Plantronics Blackwire 3225 Seri
+						Plantronics Plantronics Blackwire 3225 Seri at usb-xhci-hcd.1.auto-1.1, full sp
+	  2 [C320M          ]: USB-Audio - Plantronics C320-M
+                      Plantronics Plantronics C320-M at usb-xhci-hcd.1.auto-1.2, full speed
+
+  **USB Sound Card** - card#1:
+
+	::
+
+	  USB Offload Playback Route PCM#0        -1, -1 (range -1->255)
+
+  **USB Sound Card** - card#2:
+
+	::
+
+	  USB Offload Playback Route PCM#0        0, 1 (range -1->255)
+
+The above example shows a scenario where the system has one ASoC platform card
+(card#0) and two USB sound devices connected (card#1 and card#2).  When reading
+the available kcontrols for each USB audio device, the following kcontrol lists
+the mapped offload path for the specific device:
+
+	"USB Offload Playback Route#*"
+
+The kcontrol is indexed, because a USB audio device could potentially have
+several PCM devices.  The above kcontrols are defined as:
+
+  - ``USB Offload Playback Route PCM`` **(R)**: Returns the ASoC platform sound
+	card and PCM device index.  The output "0, 1" (card index, PCM device index)
+	signifies that there is an available offload path for the USB SND device
+	through card#0-PCM device#1.  If "-1, -1" is seen, then no offload path is
+	available for the USB SND device.
+
+USB Offload Playback Route Kcontrol
+-----------------------------------
+In order to allow for vendor specific implementations on audio offloading device
+selection, the SOC USB layer exposes the following:
+
+.. code-block:: rst
+
+	int (*get_offload_dev)(struct snd_kcontrol *kcontrol,
+			      struct snd_ctl_elem_value *ucontrol);
+..
+
+These are specific for the **USB Offload Playback Route PCM#** kcontrol.
+
+When users issue get calls to the kcontrol, the registered SOC USB callbacks will
+execute the registered function calls to the DPCM BE DAI link.
+
+**Callback Registration:**
+
+.. code-block:: rst
+
+	static int q6usb_component_probe(struct snd_soc_component *component)
+	{
+	...
+	usb = snd_soc_usb_allocate_port(component, 1, &data->priv);
+	if (IS_ERR(usb))
+		return -ENOMEM;
+
+	usb->connection_status_cb = q6usb_alsa_connection_cb;
+	usb->get_offload_dev = q6usb_get_offload_dev;
+
+	ret = snd_soc_usb_add_port(usb);
+..